1. Field of the Invention
The present invention relates to an elastic wave filter having a balanced-unbalanced conversion function and a communication device including such an elastic wave filter.
2. Description of the Related Art
To reduce the size and/or weight of communication devices, such as cellular phones, the number and size of the components included therein have been reduced in recent years. Moreover, development of components including multiple combined functions has been advanced. With such a situation as a backdrop, surface acoustic wave filters including a balanced-unbalanced conversion function, so-called the balun function, are being used in RF (radio frequency) circuits of communication devices.
However, surface acoustic wave filters including the balanced-unbalanced conversion function have a problem in that signals output from a first balanced signal terminal and those output from a second balanced signal terminal are not well balanced. Balance here refers to amplitude balance and phase balance.
International Publication No. WO2005/031971 discloses a surface acoustic wave filter that includes a balanced-unbalanced conversion function and that improves these types of balance.
FIG. 1 of International Publication No. WO2005/031971 discloses a 3-IDT-type, longitudinally coupled resonator-type surface acoustic wave filter including a balanced-unbalanced conversion function, in which first to third comb-shaped interdigital electrode units (IDTs) are disposed on a piezoelectric substrate in the surface acoustic wave propagation direction. First and second reflectors are disposed on both sides in the surface acoustic wave propagation direction of the area in which the first to third IDTs are disposed. The central second IDT includes first and second divided comb-shaped electrodes. The first and second divided comb-shaped electrodes are formed by dividing one of the comb-shaped electrodes of the second IDT into two portions in the surface acoustic wave propagation direction. The first and second divided comb-shaped electrodes are connected to first and second balanced signal terminals, respectively. The first and third IDTs are mutually connected and then connected to an unbalanced signal terminal. In International Publication No. WO2005/031971, at least one design parameter of the first to third IDTs and the first and second reflectors varies between one side and the other of a virtual central axis A extending in a direction perpendicular to the surface acoustic wave propagation direction. It is stated that this improves balance.
FIG. 13 of International Publication No. WO2005/031971 discloses a 5-IDT-type, longitudinally coupled resonator-type surface acoustic wave filter including a balanced-unbalanced conversion function, in which five IDTs are disposed in the surface acoustic wave propagation direction on a piezoelectric substrate. First and second reflectors are disposed on both sides in the surface acoustic wave propagation directions of the area in which the five IDTs are disposed. International Publication No. WO2005/031971 suggests that such a 5-IDT-type, longitudinally coupled resonator-type surface acoustic wave filter is also preferably configured so that at least one design parameter of the first to fifth IDTs and the first and second reflectors varies between one side and the other of a virtual central axis A extending in a direction perpendicular to the surface acoustic wave propagation direction, similarly to the embodiment of a 3-IDT type, longitudinally coupled resonator-type surface acoustic wave filter.
Unfortunately, International Publication No. WO2005/031971 describes nothing specific with respect to how one side and the other of the virtual central axis are different from each other in the 5-IDT-type, longitudinally coupled resonator-type surface acoustic wave filter having the balanced-unbalanced conversion function.
On the other hand, the 5-IDT-type, longitudinally coupled resonator-type surface acoustic wave filter including the balanced-unbalanced conversion function as described in FIG. 13 of International Publication No. WO2005/031971 has more configurations for making one side and the other of the virtual central axis different from each other than the 3-IDT-type, longitudinally coupled resonator-type surface acoustic wave filter including the balanced-unbalanced conversion function. Accordingly, solely making one side and the other asymmetrical with respect to the virtual central axis does not lead to a sufficient improvement in balance, and instead, balance is deteriorated.